1. Field of the Invention
The present invention relates to an electron gun arrangement suitable for use in a cathode-ray tube such as, for example, a picture tube for receiving color television.
2. Description of the Prior Art
FIG. 6 shows a conventional multibeam single electron gun arrangement, by way of example, for use in a color television receiving tube. This electron gun arrangement has cathodes K.sub.R, K.sub.G and K.sub.B corresponding to electron beams for red, green and blue, respectively. A first grid G.sub.1, a second grid G.sub.2, a third grid G.sub.3, a fourth grid G.sub.4 and a fifth grid G.sub.5 are arranged commonly for the cathodes K (K.sub.R, K.sub.G and K.sub.B). The cathodes K and the first to third grids G.sub.1 to G.sub.3 form a cathode prefocusing lens, while the grids G.sub.3 to G.sub.5 form a unipotential main electron lens. In this conventional electron gun arrangement, the electron beams respectively emitted from the cathodes K.sub.R, K.sub.G and K.sub.B intersect each other at a position substantially in the central portion of the main electron lens meeting the Fraunhofer conditions, namely, at a position which provides conditions to eliminate coma aberration. A converging means C such as, for example, electrostatic deflecting plates, is provided after the fifth grid G.sub.5 to converge the electron beams B.sub.R, B.sub.G and B.sub.B emitted from the cathodes K.sub.R, K.sub.G and K.sub.B, respectively, on a fluorescent screen, not shown. In such a conventional electron gun arrangement, since the main electron lens exerts an effect in common on all the electron beams, the aperture of the main electron lens can be increased within a limited area of the neck section of the cathode-ray tube to reduce aberration.
On the other hand, in such a cathode-ray tube, focusing conditions are decided so that the electron beams respectively emitted from the cathodes K.sub.R, K.sub.G and K.sub.B are focused on the fluorescent screen at optimum spots. Concretely, an optimum focusing voltage V.sub.F is applied to a focusing electrode, for example, the fourth grid G.sub.4 of the main electron lens of the electron gun arrangement of FIG. 6. However, the focusing conditions are different between the central portion and peripheral portion of the fluorescent screen because of the difference between the central portion and the peripheral portion have different distances from the main electron lens. Accordingly, it is a general practice to apply a dynamic focusing voltage synchronized with the horizontal and vertical deflection of the electron beams on the fluorescent screen to the focusing electrode in addition to a fixed focusing voltage V.sub.F so that the electron beams are focused satisfactorily over the entire area of the fluorescent screen.
ln an electron gun arrangement developed through the improvement of the electron gun arrangement of FIG. 6 in respect to aberration, a focusing voltage is applied to an electrode serving for both the electrode of the main electron lens and the electrode for the cathode prefocusing lens. In this improved electron gun arrangement, when a dynamic focusing voltage is applied in addition to a fixed focusing voltage V.sub.F to the cathode prefocusing lens, the cathode current varies to cause irregular brightness distribution on the fluorescent screen making the peripheral portion of the fluorescent screen brighter than the central portion.
As mentioned above, in the electron gun arrangement of FIG. 6, aberration is reduced by increasing the aperture of the main electron lens. However, the beam spot is liable to bloom due to increase in spherical aberration when the beam current is large. In the electron gun arrangement of FIG. 6, the effect of the focusing voltage for sharply focusing the electron beams at the same position on the fluorescent screen is different for the center beam B.sub.G traveling coaxially with the axis of the main electron lens L, and the side beams B.sub.R and B.sub.B traveling obliquely to the axis of the main electron lens L. That is, when the focusing voltage V.sub.F is appropriate for sharply focusing the side beams B.sub.R and B.sub.B, the center beam B.sub.G is focused at a position before the fluorescent screen and, when the focusing voltage V.sub.F is appropriate for sharply focusing the center beam B.sub.G, the side beams B.sub.R and B.sub.B are focused at a position beyond the fluorescent screen. Such a problem can be solved by disposing the central cathode K.sub.G for emitting the center electron beam away from the main electron lens L relative to the side cathodes K.sub.R and K.sub.B for emitting the side electron beams, as illustrated in FIG. 7. In the configuration shown in FIG. 7, since the second grid G.sub.2 and the successive grids are common to all the electron beams, a portion of the second grid G.sub.2 facing the first grid G.sub.1G corresponding to the central cathode K.sub.G extends backwardly so that the respective gaps between the second grid G.sub.2 and the first grids G.sub.1R, G.sub.1G and G.sub.1B respectively corresponding to the cathodes K.sub.R, K.sub.G and K.sub.B are substantially the same to cause the effect of the main electron lens on all the electron beams to be the same. However, the equipotential surfaces relating to the central beam B.sub.G are curved as indicated by lines a in FIG. 7, and the curved equipotential surfaces exert an additional focusing effect on the central beam B.sub.G. Consequently, the crossover point of the central beam is varied, the substantial object point of the electron lens system relating to the central beam is moved and, in some cases, the spot of the central beam is distorted. Such inconveniences may be avoided by curving the rear end of the third grid G.sub.3 facing the front end of the second grid G.sub.2 along the front end of the second grid G.sub.2 and by decreasing the gap between the second grid G.sub.2 and the third grid G.sub.3 so that the equipotential surfaces are parallel to each other. However, since a high voltage V is applied to the third grid G.sub.3, electrical discharges occur between the second grid G.sub.2 and the third grid G.sub.3 when the gap between the second grid G.sub.2 and the third grid G.sub.3 is too small.
Referring to FIG. 8 showing another unipotential electron lens system, a high voltage V.sub.A is applied to the third grid G.sub.3 and the fifth grid G.sub.5 of the main electron lens, and a focusing voltage V.sub.F is applied to the fourth grid G.sub.4 of the main electron lens. Ordinarily, in the unipotential electron lens system of this type, the grids G.sub.3 and G.sub.5 to which the high voltage V.sub.A is applied and the grid G.sub.4 to which the focusing voltage V.sub.F is applied are substantially the same in diameter, or as shown in FIG. 9, the respective ends of the high-voltage grids G.sub.3 and G.sub.5 facing the focusing grid G.sub.4 are reduced in diameter to shield the path of electron beams from disturbance caused by an external electric field. In either case, the focusing grid G.sub.4 is formed so as to meet a condition: l/D=0.5 to 2.0, where l is the length and D is the diameter of the grid G.sub.4.
FIG. 10 shows the calculated spherical aberration characteristics of the unipotential electron lens system comprising the grids G.sub.3 to G.sub.5 having the same diameter (FlG. 8) for various values of l/D=.psi., in which the ratio f/D=.zeta.(f=focal length, D=aperture) is measured on the X-axis and the coefficient Cs of spherical aberration is measured on the Y-axis. As is obvious from FIG. 10, when the ratio .zeta. is fixed, the coefficient Cs of, spherical aberration diminishes as the ratio .gamma. is increased. Particularly, since the value of the aperture D is limited by the diameter of the neck section of the cathode-ray tube, when the focal length f is fixed, the longer the length of the grid G.sub.4, the smaller the spherical aberration. However, since the aberration is saturated when the ratio .gamma. is 2.0 or greater, it is desirable to reduce the focal length f when the ratio .gamma. is fixed. Nevertheless, in general, when the length l of the grid G.sub.4 is increased, namely, when the ratio .gamma. is large, the focal length f cannot be diminished. This problem will be described in detail with reference to FlG. 11. ln a unipotential electron lens system, suppose that lenses 1 and 2 are formed between a third grid G.sub.3 and a fourth grid G.sub.4 and between the fourth grid G.sub.4 and the fifth grid G.sub.5, respectively, f.sub.1 and f.sub.2 are the respective object focal lengths of the lenses 1 and 2, F.sub.1 and F.sub.2 are the respective image focal lengths of the lenses 1 and 2, F.sub.1 ' and F.sub.2 ' are the respective image focal points of the lenses 1 and 2, and the distance between F.sub.1 ' and F.sub.2 ' is C. Then, the composite focal length f' is expressed by EQU f'=f.sub.1 '.times.f.sub.2 '/(-C) (1)
Generally, in the electron lens system, C&lt;0, and hence f'&gt;0. When the length l of the grid G.sub.4, hence, the distance L between the lenses 1 and 2, is increased to diminish spherical aberration, the absolute value of C decreases and, as is obvious from Eq. (1), the composite focal length f' increases. Accordingly, significant increase of l and significant reduction of f for satisfactorily reducing spherical aberration as explained with reference to FIG. 10 are incompatible. Further, increase of f causes the focusing condition to change. Accordingly, to maintain f at a small value regardless of the increase of l, as is obvious from Eq. (1), the respective image focal lengths of the lenses 1 and 2 need to be decreased. However, since the variation of the distance Q between the after lens 2 and the fluorescent screen of the cathode-ray tube is limited by the relation of the after lens 2 to the horizontal and vertical deflecting means provided at the base of the funnel of the cathode-ray tube, and the reduction of the focal length f.sub.2 ' of the after lens 2 is limited to a certain extent. Therefore, it is desired to reduce the focal length f.sub.1 ' of the front lens 1. The focal length f.sub.1 ' of the front lens 1 can be reduced, for example, by increasing the ratio of the anode voltage V.sub.A applied to the grid G.sub.3 to the focusing voltage V.sub.F applied to the grid G.sub.4, namely, V.sub.A /V.sub.F. This method, however, requires an independent high-voltage circuit for applying a high voltage to the grid G.sub.3 in addition to the circuit for the fifth grid G.sub.5, which particularly is troublesome because the high-voltage circuit requires shielding.
To reduce the focal length f.sub.1 ' of the front lens system without encountering such problems, a front electron lens (lens 1) of a deceleration type is formed of a first electrode, i.e., the third grid G.sub.3 and a second electrode, i.e., the fourth grid G.sub.4, and an after electron lens (lens 2) of an acceleration type is formed of the second electrode (fourth grid G.sub.4) and a third electrode, i.e., the fifth grid G.sub.5 as shown in FIG. 12. In this arrangement, the length l of the grid G.sub.4 is determined so that the respective electron lens regions of the front electron lens (lens 1) and the after electron lens (lens 2) are separated from each other, and the front electron lens (lens 1) and the after electron lens (lens 2) are designed so that the aperture of the front electron lens is smaller than that of the after electron lens. That is, the respective opposite ends of the third grid G.sub.3 and the fourth grid G.sub.4 are designed so that the aperture D.sub.1 thereof is smaller than the aperture D.sub.2 of the respective opposite ends of the fourth grid G.sub.4 and the fifth grid G.sub.5. That is, the grids are designed so as to comply with the inequality: D.sub.1 /D.sub.2 =k&lt;1. To separate the respective electron lens regions of the front lens and the after lens from each other, the grids are designed so as to comply with the inequalities:l.sub.1 &gt;D.sub.1, l.sub.2 &gt;D.sub.2 and l&gt;D.sub.1 +D.sub.2, where l.sub.1 is the length of the reduced section of the grid G.sub.4, l.sub.2 is the large section of the grid G.sub.4, D.sub.1 is the diameter of the reduced section of the grid G.sub.4, and D.sub.2 is the diameter of the large section of the grid G.sub.4.
Suppose that the front electron lens and the after electron lens are the same in diameter, namely, k=1, so as to form an optical system as shown by continuous lines in FIG. 13, and the electron beams are focused on the fluorescent screen S of the cathode-ray tube. In FIG. 13, P.sub.0 is an object point, namely, a cathode image formed at the cross-over point of the electron beams focused by a cathode prefocusing electron lens, P.sub.1 is a virtual image formed by the front electron lens (lens 1), namely, the object point of the after lens (lens 2), and P.sub.2 is an image focused on the fluorescent screen S by the after electron lens (lens 2). To reduce the focal length f.sub.1 ' of the front electron lens (lens 1) by decreasing the diamter D.sub.1 without varying the focusing system, namely, to maintain the focusing system so that the image is focused on the fluorescent screen S, the front electron lens (lens 1) and the object point P.sub.0 are shifted to positions indicated by broken lines, respectively. Optically, reducing the diamter of the front electron lens (lens 1) is equivalent to reducing the focusing system of the lens 1 without varying the magnification, because the respective magnifications of the lenses 1 and 2 are fixed. That is, the amount of aberration attributable to the lens 1 is expected to decrease according to the degree of reduction of the focusing system. More concretely, if the aperture D.sub.1 of the front lens 1 is decreased, the distance between the lenses 1 and 2 is increased, and the cathodes K are shifted. Supposing that M=12, Q=50.times.D.sub.2, and V.sub.F /V.sub.A is fixed, where M is the magnification of the lens system, and Q is the distance between the lens 2 and the image point P.sub.2, O is the distance between the after lens 2 and the object point P.sub.O, and L is the distance between the lenses 1 and 2, the variations of the distance O and L with the aperture ratio k are indicated by a continuous line and a broken line, respectively, in FIG. 14. In this case, the aperture D.sub.2 of the after electron lens (lens 2) is 6 mm. In FIG. 14, the distances O and L are measured on the Y-axis by the aperture D.sub.2 as a unit, namely, D.sub.2 =1.
FIG. 15 shows the calculated results of the relation between the coefficient of spherical aberration and V.sub.F /V.sub.A for the aperture ratio, where M=-8 and Q=50.times.D.sub.2. In FIG. 15, curves 10, 11, 12 and 13 represent the variations of the coefficient Cs of spherical aberration with V.sub.F /V.sub.A for k=1.0, 0.8, 0.6 and 0.4, respectively. Values in parentheses in FIG. 15 are the values of the distance L expressed by D.sub.2 as the unit. In FIG. 15, the ratio Cs/D.sub.2 is measured on the Y-axis.
FIG. 16 is similar to FIG. 15. In FIG. 16, M=-10, Q=50.times.D.sub.2, and curves 20, 21, 22, 23 and 24 are the variations of Cs with V.sub.F /V.sub.A for k =1.0, 0.8, 0.6, 0.4 and 0.3, respectively.
As is obvious from FIGS. 15 and 16, the smaller the aperture ratio k between the front and after electron lenses, namely, the smaller the aperture D.sub.1 of the front electron lens relative to the aperture D.sub.2 of the after electron lens, the greater is the improvement of aberration.
Thus, a lens system causing small spherical aberration is formed by forming an independent front lens region and an independent after lens region, and forming the front electron lens and the after electron lens so that the aperture ratio k therebetween is small. The coefficient Cs of the total spherical aberration of the composite lens system consisting of the lenses 1 and 2 formed by the front and after lens regions is expressed by EQU Cs=k.multidot.Cs.sup.1 +1/M.sub.1.sup.4 .multidot.(V.sub.1 /V.sub.2).sup.3/2 .multidot.Cs.sup.2 ( 2)
where Cs.sup.1 and Cs.sup.2 are the coefficients of spherical aberration of the lenses 1 and 2, respectively.
Therefore, the amount of aberration r is EQU .DELTA.r=M.sub.1 .multidot.M.sub.2 .multidot.{k.multidot.Cs.sup.1 (.phi..sub.1)+1/M.sub.1.sup.4 .multidot.(.phi..sub.1).sup.3/2 .multidot.Cs.sup.2 (.phi..sub.2)}.times.(1/.phi..sub.0).sup.3/2 .multidot..alpha..sub.0.sup.3 ( 3)
where k is the aperture ratio of the aperture D.sub.1 of the front electron lens to the aperture D.sub.2 of the after electron lens, M.sub.1 and M.sub.2 are the respective magnifications of the front and after electron lenses, .phi..sub.0 =V.sub.1 /V.sub.3, .phi..sub.1 =V.sub.2 /V.sub.1, .phi..sub.2 =V.sub.2 /V.sub.3, and V.sub.1, V.sub.2 and V.sub.3 are voltages applied to the first, second and third electrodes, respectively.
It is obvious from Equation (3) that reducing the aperture ratio k is effective for reducing the total aberration.
Thus, the aberration characteristics of the main electron lens consisting of the two independent lenses 1 and 2 can be improved by designing the lenses 1 and 2 so that the aperture ratio k is small, however, such a main electron lens is unsatisfactory with regard to astigmatism and the curvature of field. Accordingly, even if such a composite lens system is employed as a common main electron lens for a plurality of electron beams, for example, three electron beams as previously explained with reference to FIG. 6, and is designed so that the three beams B.sub.R, B.sub.G and B.sub.B will intersect each other at a position to make coma aberration zero to meet the Fraunhofer conditions, the respective spots of the side beams B.sub.R and B.sub.B are liable to bloom.
Japanese Patent Provisional Publication No. 55-19755 discloses an electron gun arrangement intended to improve the condition of the spots of the side beams. In this known electron gun arrangement, a main electron lens comprises front electron lenses, namely, a front electron lens regions, and an after electron lens separate from the front electron lenses, namely, an after electron lens region. The front electron lenses, in particular, are individual electron lenses corresponding to the electron beams, respectively, while the after electron lens is a common electron lens for all the electron beams, having small characteristics of astigmatism and curvature of field. The aperture of each front electron lens is smaller than that of the after electron lens. In this electron gun arrangement, for example, the electrodes forming the front electron lenses of the main electron lens serve also as the electrodes of the cathode prefocusing electron lens, and hence the same focusing voltage is applied to those electrodes, whereby the cathode current is caused to vary by the dynamic focusing voltage, and the brightness of the fluorescent screen is caused to vary from position to position.
As illustrated in FIG. 17, this known electron gun arrangement has, for example, a main electron lens comprising front electron lenses of an decelerating bipotential electron lens system and an after electron lens of an accelerating bipotential electron lens system. Cathodes K.sub.R, K.sub.G and K.sub.B for emitting, for example, electron beams B.sub.R, B.sub.G and B.sub.B for red, green and blue, respectively, are provided and first grids G.sub.1R, G.sub.1G and G.sub.1B, second grids G.sub.2R, G.sub.2G and G.sub.2B, and third grids G.sub.3R, G.sub.3G and G.sub.3B for the electron beams B.sub.R, B.sub.G and B.sub.B, respectively, are arranged sequentially. Fourth and fifth grids G.sub.4 and G.sub.5, namely, common grids, are arranged sequentially after the third grids. One end of the fourth grid G.sub.4 facing the third grids G.sub.3R, G.sub.3G and G.sub.3B is trifurcated in three cylindrical electrodes G.sub.4R, G.sub.4G and G.sub.4B, respectively, corresponding to the third grids G.sub.3R, G.sub.3G and G.sub.3B. Voltages according to an inequality V.sub.2 &lt;V.sub.1 &lt;V.sub.3, where V.sub.1 is a voltage applied to the third grids, V.sub.2 is a voltage applied to the fourth grid and V.sub.3 is a voltage applied to the fifth grid, are applied to the third, fourth and fifth grids, respectively. For example, the voltages V.sub.1 and V.sub.3 are equal to an anode voltage V.sub.H. The electrodes G.sub.4R, G.sub.4G and G.sub.4B of the fourth grid G.sub.4 and the third grids G.sub.3 constitute the decelerating bipotential front electron Lens.sub.1R, Lens.sub.1G and Lens.sub.1B individually for the beams B.sub.R, B.sub.G and B.sub.B, respectively, of a main electron lens, while the fourth grid G.sub.4 and the fifth grid G.sub.5 constitute an accelerating bipotential after electron lens 2 commonly for the beams B.sub.R, B.sub.G and B.sub.B. The aperture ratio k of the aperture D.sub.1 of the electrodes G.sub.4R, G.sub.4G and G.sub.4B of the fourth grid G.sub.4 to the aperture D.sub.2 of the fourth grid G.sub.4 at one end thereof facing the fifth grid G.sub.5 is smaller than one, namely, k=D.sub.1 /D.sub.2 &lt;1. The length of the fourth grid G.sub.4 is greater than D.sub.1 +D.sub.2 to separate the lens region of the after electron lens 2 from the lens region of the front Lens.sub.1R, Lens.sub.1G and Lens.sub.1B. The electron beams B.sub.R, B.sub.G and B.sub.B are caused to intersect each other substantially at the center of the after electron lens 2 so as to meet the Fraunhofer conditions.
Referring to FIG. 18, showing another conventional electron gun arrangement, the after electron lens 2 of this electron gun arrangement is an extension type (extended field type) bipotential electron lens. The after electron lens 2 comprises fourth, fifth and sixth grids G.sub.4, G.sub.5 and G.sub.6, and voltages V.sub.1 to V.sub.4 applied to the third grids G.sub.3, the fourth grid G.sub.4, fifth grid G.sub.5 and the sixth grid G.sub.6 meet, for example, the following conditions: V.sub.1 =V.sub.4 =anode voltage, V.sub.2 /V.sub.1 =0.25 to 0.40, and V.sub.3 /V.sub.4 =0.4 to 0.6.
The forming the main electron lens of the front electro lenses respectively for the electron beams, each having a small aperture, and the after electron lens commonly for all the electron beams, having a large aperture solves the problems of astigmatism of the front electron lenses and those of the curvature of field, and enables the employment of an electron lens having small astigmatism and the curvature of field as the after electron lens; consequently such a main electron lens of a multibeam single electron gun type is able to solve the blooming of the spots of the side beams attributable to astigmatism and the curvature of field.
FIG. 19 illustrates the electrode configuration of the foregoing electron gun arrangement. First grids G.sub.1R, G.sub.1G and G.sub.1B are provided for cathodes K.sub.R, K.sub.G and K.sub.B, respectively, while second to sixth grids G.sub.2 to G.sub.6 are common grids. Thus, front lenses Lens.sub.1R, Lens.sub.1G and Lens.sub.1B of a main electron lens are provided for electron beams emitted from the cathodes K.sub.R, K.sub.G and K.sub.B, respectively. The lenses Lens.sub.1R, Lens.sub.1G and Lens.sub.1B are formed of electron beam transmission apertures h.sub.3R, h.sub.3G and h.sub.2B formed in the front end plate of the common third grid G.sub.3, and electron beam transmission apertures h.sub.4R, h.sub.4G and h.sub.4B formed in the front end plate of the common fourth grid G.sub.4, respectively. Similarly, to the foregoing front electron lenses, the front electron lenses Lens.sub.1R, Lens.sub.1G and Lens.sub.1B are formed to meet the required relation described above. In forming the front electron lenses, the electron beam transmission apertures h.sub.3R, h.sub.3G and h.sub.3B formed in the front end plate of the common third grid G.sub.3 and the electron beam transmission apertures h.sub.4R, h.sub.4G and h.sub.4B formed in the front end plate of the common fourth grid G.sub.4 are formed with a press to form cylindrical walls Ws around the apertures, respectively, to prevent mutual disturbance in the respective electric fields. Electron beam transmission apertures are formed in the respective front end plates of the first grids G.sub.1R, G.sub.1G and G.sub.1B, the second grid G.sub.2 and the third grid G.sub.3 to form cathode prefocusing lenses respectively for the electron beams. The electron beam transmission apertures forming the cathode prefocusing lenses and the front electron lenses are coaxial with axes O.sub.R, O.sub.G and O.sub.B, which are in alignment with the electron beams, respectively. The axes O.sub.R and O.sub.B corresponding to the side beams are inclined at a predetermined angle .theta. to the axis O.sub.G corresponding to the center electron beam. Accordingly, the cylindrical walls Ws formed around the electron beam transmission apertures h.sub.3R, h.sub.3G and h.sub.3B formed in the front end plate of the third grid G.sub.3 and the electron beam transmission apertures h.sub.4R, h.sub.4G and h.sub.4B formed in the rear end plate of the fourth grid G.sub.4 should be formed coaxially with the axes O.sub.R, O.sub.G and O.sub.B, respectively, which requires complicated manufacturing processes and tends to cause problems during manufacturing accuracy such as maintaining the axes at accurate positions.